1. Field of the Invention
The present invention relates to a slit exposure projection device used in a copying machine or the like.
2. Description of the Prior Art
In the past, a single lens or compound eye condensing light transmitting medium has been used as a focusing optical system of a copying machine. The single lens is a conventional transmissive lens or mirror type lens which may be a fixed focus lens or variable focus lens. It is of a circular shape and the aperture diaphragm (entrance pupil and exit pupil) thereof is also circular.
In recent years, size reduction and high performance of the copying machine have been desired. U.S. Pat. No. 3,504,960 shows a slit exposure copying machine which uses a parallel stop external to of a conventional focusing lens having a round pupil to improve a aberration in the lateral direction (sagittal direction) of the exposure slit. This technique improves the performance of the focusing lens but does not reduce the size of the device and the cost is high because of arrangement of the stop external to the focusing lens. Further, because of the arrangement to the stop external of the lens, a half shade image of a light flux on the photo-receiving plane is complex.
Photograph Industry, 41(81) pages 115-120 (1983) discloses an oval lens having a truncated spherical lens. Since the field angle necessary in the lateral direction of the exposure slit (short direction of the slit) is smaller than the field angle necessary in a longitudinal direction of the slit (long direction of the slit), if a circular lens about an optical axis is used, a light flux directed from the pupil of the lens to a photo-receiving plane through the slit does not pass through the area of the lens away from the optical axis of the lens in the slit lateral direction. This lens area is cut off to form the oval lens as shown in the photograph Industry reference. Accordingly, the pupil of this lens is circular and the light flux transmitted through the pupil is not eclipsed by the lens and reaches the photo-receiving plane.
However, the prior art device still has a problem in providing a high performance slit exposure projection device which allows size reduction and cost reduction of the device while maintaining sufficient lens performance and F-number.